WO 2004/051303 A1 discloses a method and a system for monitoring and position determination of objects and/or living beings within an area, such as, e.g. a room in a building or a road tunnel. The system comprises a plurality of identification tags equipped with an ultrasonic receiver and radio transmitter which is attached to the objects that have to be monitored. The identification tags receive ultrasonic signals whose arrival time they measure. This information together with the identification tags' ID code are transmitted by radio waves to a central unit which calculates the position of each of the identification tags.
It is a disadvantage of the known system that it does not offer sufficiently high spatial accuracy to provide true access/exit control, nor sufficiently high temporal accuracy to allow for immediate responses to critical tracking events.
European patent application EP 2 469 298 A1 in the name of the present applicant, discloses a method and device for determining location of a target, using mobile tags and fixed beacons, wherein the target location is estimated from a weighted sum of the indications of the received signal power of at least two received signals. It is a disadvantage of this system that multiple received signals must be available to perform RSSI triangulation.